I. Field of the Invention
Method of correcting and maintaining proper carbon balance in hard metals consisting of various carbides such as WC, TiC, TaC, HfC, MoC or mixtures thereof and various metal binders such as Co, Fe, Ni or mixtures thereof in a carbon containing furnace.
II. Description of the Prior Art
The carbon content of hard metals such as WC--Co alloys must be controlled within very narrow limits in order to optimize the performance of these materials. Much work has been done which shows the detrimental effect on performance of WC--CO alloys which are deficient in carbon or contain an excess of carbon. FIG. 1 shows an isothermal section of the ternary W--C--Co phase diagram at 1400 degrees Centigrade. The region reflecting a proper carbon balance for the desired two phase structure (WC--Co) is that bounded by the phase lines on either side of the dotted line connecting the compound WC and the cobalt corner of the diagrams. If the carbon content deviates over the WC--Co phase line toward the carbon side of the diagram, free carbon will form in the microstructure. If the carbon content deviates below the WC--Co phase line toward the tungsten side of the diagram, eta phase will form in the microstructure. Formation of either of these phases has been shown to be extremely detrimental to the performance of carbide alloys.
The boundaries of these phase lines have been calculated by Suziki (1) to be approximately given by the following formula:
Upper carbon rich boundary 6.13-0.058.times.wt % Co PA0 Lower carbon deficient boundary 6.13-0.079.times.wt % Co
It it also known that even within the two phase field (WC--Co), the properties of the material can vary a great deal with carbon content. In order to optimize properties it is generally accepted that carbon should be held to within + or -0.03% carbon of mid-range for a given alloy.
A great deal of work has been previously done to develop processes and techniques at all production stages of WC--Co alloys such that in the final sintered product, the carbon content will end up in the proper range for a given alloy, i.e. neither carbon deficient (eta phase) nor excessive carbon. Many of these previously known processes are conducted under cover gasses or liquids to prevent powder oxidation which is both difficult and time consuming. Other techniques include extensive carbon analysis of the powder and the addition of carbon or tungsten as required to obtain the desired percentage of carbon content. Even with all of the above precautions and many more, materials out of carbon balance are still produced and routine control to obtain the desired carbon content, .+-.0.03%, cannot be accomplished.
There have been a number of previously known methods to increase the carbon content in carbon deficient parts and vice versa. For example, G. E. Spriggs: Powder Metall., 1960, 7, 296, has shown that during presintering in hydrogen, complete protection from decarburization by the reaction WC+WH.sub.2 .revreaction.W+CH.sub.4 at 800.degree. Centigrade may be accomplished by the addition of 2% by volume of CH.sub.4 to the hydrogen. This has been confirmed by S. Takatsu: Powder Metall., Int., 1978, 10, 1, 13, and others.
Gortsema and Kotval: Planseebar, Pulvermetall., 1976, 24,254, and S. Takatsu found that when attempting to carburize WC--Co powders at 750 to 925 degrees Centigrade with various methane additions at 1% to 5% volume, it was difficult to maintain close control of the carbon under atmospheric conditions.
Nissenhalts and Barts, Z. Nissenhalts and J. Barts: Planseeber, Pulvermetall., 1974, 22, 81, previously ran experiments in a carbon free furnace to obtain a thermodynamic equilibrium of the carbon content of WC--Co alloys and a hydrocarbon--H2 gas mixture, namely with a methane, cooking gas or toluene mixture. The materials were exposed to these gaseous mixtures at sintering temperature (1400 degrees C.) and it was found that exact carbon control was difficult to achieve.
Other prior art publications include L. Suzuki and H. Kubota: Planseeber, Pulvermetall., 1966, 14, 96, W. J. Moore: Physical Chemistry., 1962, E. Horvath, Bundesrepublik Deutschland Pat. No. 22 33 852, P. Rautala, J. Norton, Trans. AIME 194, 1045 91952.
None of these previously known methods for correcting the carbon content of carbide preforms, however, have been capable of consistently holding carbon content to .+-.0.032% or simultaneously correcting carbon deficient and carbon excessive parts.